1. Field of the Invention
The present invention relates to the transfer of different data types in a telecommunications system, and more particularly to the routing of such data between a network connectable to the telecommunications system and subscriber terminals of the telecommunications system.
2. Description of the Prior Art
In a typical telecommunications system, a subscriber terminal may be located at a subscriber""s premises for handling transfer of data to and from that subscriber. One or more lines may be provided from the subscriber terminal for supporting one or more items of telecommunications equipment located at the subscriber""s premises. Alternatively, the subscriber terminal may be an integral part of the item of telecommunications equipment. Further, a central terminal may be provided for controlling a number of subscriber terminals, and in particular for managing transfer of data between a subscriber terminal and other components of a telecommunications network.
Each subscriber terminal communicates with the central terminal via a transmission medium, for example copper wires, optical fibres, etc for a wired system, or some form of radio resource for a wireless system. In accordance with known techniques, multiple communication channels may be arranged to utilise the transmission medium for the transmission of signals to and from the subscriber terminal. For example, in a xe2x80x9cCode Division Multiple Accessxe2x80x9d (CDMA) system, signals may be transmitted over the transmission medium on a particular frequency channel, and this frequency channel may be partitioned by applying different orthogonal codes to signals to be transmitted on that frequency channel. Signals to which an orthogonal code has been applied can be considered as being transmitted over a corresponding orthogonal communication channel utilising the particular frequency channel. Similarly, in a xe2x80x9cTime Division Multiple Accessxe2x80x9d (TDMA) system, a particular frequency channel can be partitioned in the time domain, such that a number of different signals can be transmitted in different time slots, the time slots forming multiple communication channels utilising the particular frequency channel. As another example, in a xe2x80x9cFrequency Division Multiple Accessxe2x80x9d (FDMA) system, a band of frequencies may be partitioned to form a number of communication channels at particular frequencies, thereby enabling multiple signals to be transmitted over the transmission medium.
Traditionally, such telecommunications systems have been used to handle voice calls to and from the subscriber terminals, and transport mechanisms have been developed for routing the voice data for such voice calls through the telecommunications system in an efficient manner.
However, nowadays, there is an ever increasing demand for such telecommunications systems to be able to transmit different types of data, for example Internet data, leased line data, basic rate ISDN data, etc., in addition to, or instead of, voice data. Whichever transport mechanism is chosen for the telecommunications system will generally be more efficient for certain data types than for others, given the differences between the types of data.
As the demand for data to be transmitted at higher and higher speeds increases, it is becoming desirable to provide a telecommunications system which facilitates more efficient transmission of data through the telecommunications system.
Viewed from a first aspect, the present invention provides a telecommunications system for connecting to a network and for routing data of a plurality of different data types between the network and subscriber terminals of the telecommunications system, the subscriber terminals being connectable to a central terminal of the telecommunications system via a transmission medium, the telecommunications system providing a number of communication channels arranged to utilise the transmission medium for transmission of data between the central terminal and the subscriber terminals, the telecommunications system comprising: a transmitter having first transmission processing logic for employing a first transport mechanism to transmit data and second transmission processing logic for employing a second transport mechanism to transmit data; a switching element for routing data for transmission to either the first or second transmission processing logic dependent on first predetermined criteria, the first predetermined criteria comprising at least the data type of the data for transmission; and resource allocation logic for determining based on second predetermined criteria which of the communication channels to allocate for use by the first transmission processing logic and which of the communication channels to allocate for use by the second transmission processing logic.
Typical known telecommunications systems utilise the same transport mechanism for handling transfer of data irrespective of its data type. Often, the transport mechanism has been developed with voice data in mind. However, unlike voice calls which are delay sensitive and thus require continuous operation and relatively constant bit rates, transfer of certain other data types (e.g. Internet data) is often bursty, and typically is not delay sensitive, and accordingly the transport mechanisms provided for handling voice calls are often not particularly efficient at handling transfer of other types of data.
In accordance with the present invention, a transmitter is provided that has both first transmission processing logic for employing a first transport mechanism to transmit data, and second transmission processing logic for employing a second transport mechanism to transmit data. A switching element is then provided to route data for transmission to either the first or second transmission processing logic dependent on first predetermined criteria, such as data type of the data for transmission. Preferably, there will be a predetermined relationship between the data type and the most appropriate transport mechanism to be used for that data type, and accordingly, unless other predetermined criteria dictate otherwise, the switching element will route the data to the first or second transmission processing logic based on whether data of that data type is best transmitted using the first transport mechanism or the second transport mechanism, respectively.
Hence, as an example, the first transport mechanism may be more suited for data types that form continuous data sequences, for example voice data, or leased line data, whereas the second transport mechanism may be more suited to more bursty data, such as Internet Protocol (IP) data. Further, it should be noted that certain forms of data, such as ISDN data, may actually have more than one basic data type. For example, ISDN data can either be sent in a packet mode, or a continuous mode, and hence there will preferably be at least two data types for ISDN data to reflect the two different ISDN modes.
In addition to specifying the data types relatively coarsely based on their basic type, e.g. IP, voice, ISDN packet mode, etc, further parameters can be taken into account in order to specify data types at a finer granularity. For example, a number of data types may be specified using predetermined parameters, and then the corresponding parameters will be taken into account for any data to be routed by the switching element. Thus, as an example, a number of different data types for IP data may be specified using a priority parameter coded as follows:
Other parameters that may be used to define different data types are tolerance to absolute delay, tolerance to delay variation, and tolerance to packet loss (bit error rate). Voice/video services generally have a low tolerance to all of the above parameters, whereas best effort IP data is generally more tolerant to all of the above parameters. It will be appreciated that the above identified parameters are merely examples of parameters that may be used to define data types, and that various other parameters could alternatively be used.
Additionally, in accordance with the present invention, resource allocation logic is provided for determining based on second predetermined criteria which of the communication channels to allocate for use by the first transmission processing logic and which of the communication channels to allocate for use by the second transmission processing logic. In preferred embodiments, the second predetermined criteria are chosen such that the allocation of communication channels to the first or second transmission processing logic can be altered dynamically.
By the above approach, a very flexible transmission mechanism is provided, which provides some choice as to the transport mechanism used to transmit any particular type of data, with the aim of improving the efficiency of transmission of data through the telecommunications system. Furthermore, the provision of the resource allocation logic enables the efficiency to be further improved by enabling allocation of the communication channels to either the first transmission processing logic or the second transmission processing logic to be altered during use with the aim of increasing throughput of data.
Although the invention requires the provision of more circuitry than a system using a single transport mechanism, it has nevertheless been found that this is more than compensated for by the significant performance benefits that can be yielded using the present invention.
It will be appreciated by those skilled in the art that in addition to data type of the data for transmission, certain other factors may be included within the first predetermined criteria applied by the switching element to determine which transmission processing logic to route data to in any particular instance. In preferred embodiments, the first predetermined criteria further comprises information indicating the transport mechanisms supported by a destination device for the data, the switching element being arranged, if the destination device only supports one of the transport mechanisms, to select the transmission processing logic employing the supported transport mechanism, but otherwise to select the transmission processing logic based on the data type of the data for transmission.
It will further be appreciated by those skilled in the art that the second predetermined criteria applied by the resource allocation logic may take a variety of forms. However, in preferred embodiments, the second predetermined criteria specifies one of the first and second transmission processing logic as having the higher priority but by default allocates the communication channels to the other of said first and second transmission processing logic. Hence, as an example, the second transmission processing logic may be allocated all of the communication channels in the absence of any transmission activity by the first transmission processing logic. However, as soon as the first transmission processing logic has data to send, the resource allocation logic will allocate one or more communication channels to the first transmission processing logic, irrespective of the demand placed on the second transmission processing logic. When the demands placed on the first transmission processing logic decrease, the resource allocation logic will then reallocate communication channels to the second transmission processing logic.
In preferred embodiments, the first transmission processing logic has the advantage of exhibiting low delay ( less than 1 ms one way), and constant bit rate. However, it exhibits only moderate spectral efficiency, and there is a significant time overhead for call set-up, clear down (xcx9c500 ms). In contrast, the second transmission processing logic of preferred embodiments has the advantage of exhibiting highest spectral efficiency, and fast packet multiplexing. Variable bit rates are used for transmission, and moderate delay is incurred (12-16 ms one way).
In accordance with the preferred embodiment described above, in the absence of demand all channels are allocated to the non-preferred transport mechanism (the second transmission processing logic). When demand for the preferred transport mechanism (the first transmission processing logic) occurs, channels are reallocated. Channels are then reallocated to the non-preferred transport mechanism in response to lowered demand on the preferred transport mechanism. The effect of reallocating channels to the first transmission processing logic is to lower throughput and increase the delay experienced by users of the second transmission processing logic. The main advantage of this approach is its simplicity. The first transmission processing logic of preferred embodiments operates as a connection based medium so demand is readily measured by analysing call set-up and clear-down events.
As an alternative to the above approach for resource allocation, the second predetermined criteria may comprise priority information identifying relative priorities for different data types. Through this approach, the resource allocation logic can be arranged to allocate communication channels with the aim of optimising quality of service (QoS) obligations. The resource allocation logic can be arranged to maintain as the priority information a database of QoS targets per user per data type. QoS may be defined in a number of different ways depending on data type, e.g. for a voice call it may be the percentage call success rate, for IP it may be a lower bound on bit rate measured in bits per second. It will be appreciated that more complex measures are also possible. At any point in time the system is carrying traffic and is subject to new demand. For each active traffic link the resource allocation logic can be arranged to measure actual QoS versus target QoS and to use this information to generate a weighted score which represents how well the overall QoS target is being met. The resource allocation logic can then adjust channel usage to maximise this score.
As a further alternative approach to handling resource allocation, the second predetermined criteria may comprise information about demand placed on the first and second transmission processing logic. By this approach, resource can be allocated with the aim of maximising throughput and spectral efficiency. Hence, for example, under lightly loaded conditions channels could be allocated as requested by the switching element. When load increases to a point where all channels are used, the resource allocation logic is then arranged to recognise that one of the transport mechanisms may be more spectrally efficient at the expense of another parameter, e.g. delay. Hence, as an example, voice calls which would otherwise be transmitted via the first transmission processing logic may be converted to xe2x80x9cVoice over IPxe2x80x9d (VOIP) data to be transmitted via the second transmission processing logic.
Given that the present invention provides for the use of two different transport mechanisms, then in preferred embodiments the telecommunications system further comprises: a receiver having first reception processing logic for processing data transmitted using the first transport mechanism and second reception processing logic for processing data transmitted using the second transport mechanism; and channel switching logic for routing data to either the first or second reception processing logic dependent on the communication channel upon which the data is received.
Hence, in preferred embodiments, the receivers can process data transmitted using either transport mechanism, thus maximising the benefits available through use of the present invention. Nevertheless, backward compatibility with pre-existing receivers is still maintained in preferred embodiments, since as mentioned earlier the switching element will preferably take account of the transport mechanisms supported by a particular receiver when determining which transmission processing logic to route data to that is destined for that particular receiver.
In preferred embodiments, the first transport mechanism employed by the first transmission processing logic is a transport mechanism designed for transmitting data types that form continuous data sequences. Hence, in preferred embodiments, voice data will preferably be routed via the first transmission processing logic, as will other data types that form continuous data sequences.
In preferred embodiments of the present invention, the second transport mechanism employed by the second transmission processing logic is a transport mechanism designed for transmitting data messages.
The term xe2x80x9cdata messagexe2x80x9d as used herein refers to a discrete entity of data to be transmitted, and will vary dependent on the data type being transmitted. For example, for Internet Protocol (IP) data, a data message would typically be an IP data packet (this being of variable length). Similarly, for ISDN data, when operating in packet mode, a data message would typically be an ISDN data packet. For data types that form continuous data sequences, for example voice data, leased line data, or ISDN calls (when operating in continuous mode), data messages may be formed by packetizing the data sequence into data messages.
From the above description, it can be seen that data messages can be formed even from data of the type that forms continuous data sequences. However, preferably, data types that form continuous data sequences are sent via the first transmission processing logic, whilst data types that form discrete data entities, such as IP data, are sent via the second transmission processing logic.
Considering the second transmission processing logic of preferred embodiments, the transmitter is preferably provided within the central terminal for transmitting a data message destined for a particular subscriber terminal over at least one of the communication channels as a number of data blocks, and the second transmission processing logic preferably comprises a frame generator for generating a number of frames to represent each data block, each frame having a header portion and a data portion, the header portion being arranged to be transmitted in a fixed format chosen to facilitate reception of the header portion by each subscriber terminal and being arranged to include a number of control fields for providing information about the data portion, the data portion being arranged to be transmitted in a variable format selected based on third predetermined criteria relevant to the particular subscriber terminal to which the data portion is destined.
In accordance with this embodiment of the present invention, the header portion is transmitted in a fixed format chosen to facilitate reception of the header portion by each subscriber terminal, and is arranged to include a number of control fields for providing information about the data portion. The choice of such a fixed format enables each subscriber terminal to receive the header portion, and hence in preferred embodiments the header portion can include information to enable each subscriber terminal to determine whether the corresponding data portion is destined for that subscriber terminal or not. This provides a particularly efficient technique for notifying subscriber terminals whether a data portion is destined for them, without the need for any subscriber terminal to begin processing the data portion itself in order to determine that information. Any subscriber terminals to which the data portion is not destined can merely ignore the data portion provided in that frame, thereby leaving the data portion to be processed by the subscriber terminal(s) to which that data portion is destined.
In contrast, the data portion is in this embodiment arranged to be transmitted in a variable format selected based on third predetermined criteria relevant to the particular subscriber terminal to which the data portion is destined. Accordingly, based on this third predetermined criteria, a variable format can be selected which is aimed at optimising the efficiency of the data transfer to the subscriber terminal.
Generally, the more efficient data formats, i.e. those that enable higher bit rates to be achieved, are less tolerant of noise. Hence, if there is a good quality communication link with a subscriber terminal, it should be possible to use a more efficient format for the data portion than may be possible if the communication link were of poorer quality. Accordingly, in preferred embodiments, the third predetermined criteria which affects the selection of the variable format comprises an indication of the signal-to-noise ratio (SNR) of signals received by the destination subscriber terminal from the central terminal. Alternatively, or in addition, the third predetermined criteria may comprise an indication of the bit error rate and/or signal strength of signals received by the destination subscriber terminal.
In preferred embodiments, the signal-to-noise ratio will determine which data formats may be used for the data portion of the frame. Given the available data formats, the amount of data in the data message will then be considered. It is generally desirable to minimise the number of blocks required to send the data message, as this will reduce the noise generated through the transmission of that data message. Hence, if one of the available data formats enables the data message to be transmitted in less blocks than any other available data format, then preferably that data format will be chosen for the one or more data frames in that data block. However, assuming a plurality of formats are available which can transmit the data in the same number of blocks, e.g. one block, then the choice of format is chosen to minimise transmission power. By minimising the transmission power, the effect of the transmission on other blocks being transmitted will be minimised.
In other words, in preferred embodiments, if there are a plurality of formats selectable as the variable format given the indicated signal-to-noise ratio and the amount of data to be sent in the data block, then the frame generator is arranged to select from those plurality of formats the format requiring lowest transmission power.
In preferred embodiments, the variable format for the data portion is defined by a number of parameters, a first parameter being a channel coding to be applied to the data in the corresponding data portion. The channel coding may, for example, be convolutional encoding used for Forward Error Correction (FEC) encoding of the data. The convolutional encoding rate may be altered depending on the quality of the communication link. Hence, as an example, a rate of xc2xd (i.e. one bit decoded for every two bits of the encoded signal) may be used for lower quality links, whereas a rate of xc2xe (i.e. three bits decoded for every four bits of the encoded signal) may be used for better quality links.
In preferred embodiments, a second parameter used to define the variable format is a modulation type to be applied to the data in the corresponding data portion. For example, different types of modulation, e.g. QAM64, QAM16, or QPSK, can be used dependent on the quality of the communication link with the subscriber terminal. Unlike voice, which typically requires constant data rate, bursty data can take advantage of variable modulation to improve efficiency.
Finally, in preferred embodiments, a third parameter used to define the variable format is a symbol rate for the data in the corresponding data portion. The symbol rate may be varied in a number of ways. For example, in a CDMA system, the symbol rate can be altered by changing the spreading gain. For a constant chip rate, the spreading gain is inversely proportional to the symbol rate. Hence, a lower spreading gain will be generally give rise to a higher symbol rate, and so lower spreading gains will typically be used for better quality communication links.
Of course, it will be appreciated by those skilled in the art that there is no requirement to use all three of the parameters identified above, and in alternative embodiments any combination of those parameters, along with any other suitable parameters (e.g. a signal constellation modifier for modifying the peak-to-mean ratio), may be used to define the variable format.
In preferred embodiments, the parameters defining the variable format used for the data portions are identified in one or more control fields of the corresponding header portion, whereby the processing circuitry within the subscriber terminal can obtain the necessary information about the format prior to processing the data portion.
As mentioned previously, the fixed format chosen for the header portion is aimed at facilitating reception of the header portion by each subscriber terminal. It will be appreciated by those skilled in the art that a number of formats may be developed for that purpose. However, in preferred embodiments, the fixed format employs a relatively low symbol rate which enables accurate reception of the header field without the need for any FEC decoding. Accordingly, in preferred embodiments, the fixed format used for the header portion employs no channel coding, although it will be appreciated that the addition of channel coding would further improve reception accuracy but at the expense of increased complexity of the reception circuitry. Furthermore, the header portion is preferably transmitted at a constant period and rate.
In preferred embodiments, each subscriber terminal comprises a first number of channel monitors to enable each of the communication channels to be monitored, whereby each subscriber terminal can read the header portion of each frame irrespective of which communication channel that frame is transmitted on. Hence, as an example, if sixteen communication channels are provided, then sixteen frames can be simultaneously transmitted over the transmission medium, and each subscriber terminal is able to the read the header portion of each of those sixteen frames.
Further, in preferred embodiments, the header portion includes an identification field identifying the subscriber terminal for which the corresponding data portion of the frame is destined, each subscriber terminal comprising a second number of processors for processing data portions destined for that subscriber terminal based on information about the variable format identified in the control fields of the corresponding header portion, and the channel monitors being arranged to identify to the processors those frames containing data portions destined for that subscriber terminal.
Hence, by the above approach, data destined for any subscriber terminal can be transmitted in a frame on any of the communication channels, and the subscriber terminal will identify those frames containing data portions destined for it, with the appropriate frames then being passed on to the processors within the subscriber terminal for processing of the corresponding data portions.
Since the header portions are transmitted in a fixed format facilitating reception of those header portions by each subscriber terminal, the channel monitor can be formed by a simple pre-processing element, which is relatively cheap and compact. Accordingly, it is perfectly acceptable to provide a channel monitor for each of the communication channels. However, since the data portions are transmitted in a variable format, and are typically channel coded, significant processing circuitry is required to decode the data portions, and it will generally not be cost effective to provide such processing circuitry for each communication channel, since in most implementations the per subscriber data rate will be only a fraction of the transmission medium data rate.
Accordingly, in preferred embodiments, the second number of processors are less than the first number of channel monitors, whereby at any point in time the header portions of the frames on each of the communication channels can be read, but only the second number of data portions can be processed by a particular subscriber terminal. By this approach, even though any particular subscriber terminal can only decode up to the second number of data portions at any one time, the central terminal has complete flexibility as to which communication channel data portions destined for that subscriber terminal are transmitted in, thus enabling the central terminal to make efficient use of the available resources of the transmission medium. In certain embodiments, some of the communication channels may exhibit better signal-to-noise ratios than other communication channels, and accordingly this flexibility can be used to make use of the communication channels that facilitate the use of the more efficient format.
The previous description of the preferred embodiment of the second transport mechanism has concentrated on the transmission of data on a downlink communication path from the central terminal to the subscriber terminal. However, in preferred embodiments, the frame format can be used for transmission of data on an uplink communication path from the subscriber terminal to the central terminal. Accordingly, in preferred embodiments, the frame generator is also provided in at least one of the subscriber terminals to enable frames to be generated for data blocks to be transmitted from the subscriber terminal to the central terminal, the subscriber terminal being arranged to issue to the central terminal over the transmission medium a request signal when it has data to send to the central terminal, the central terminal being responsive to the request signal to grant access to the subscriber terminal on a communication channel selected by the central terminal.
It will be appreciated that certain of the fields provided within the header portion of preferred embodiments are in theory not required for an uplink frame. For example, the identification field used in preferred embodiments to identify the destination subscriber terminal may be redundant assuming that there is only one central terminal to which the uplink frame can be sent. In that event, in preferred embodiments, any such fields can be used in the uplink frame to contain any uplink specific protocol information. For example, such control fields may be used to indicate the number of blocks or frames remaining for the subscriber terminal to send.
In preferred embodiments, the central terminal manages allocation of communication channels to the subscriber terminals for sending frames from the subscriber terminals to the central terminal, to avoid any contentious access by multiple subscriber terminals to the same communication channel.
It will be appreciated by those skilled in the art that there are a number of different approaches that may be used for avoiding such contentious accesses. However, in preferred embodiments the central terminal is arranged to grant access by including in a control field of a frame issued by the central terminal on the selected communication channel a grant signal identifying the subscriber terminal. This approach is possible due to the fact that each subscriber terminal will receive each header portion of the frame, irrespective of which communication channel it is sent on. Accordingly, the relevant subscriber terminal will identify the grant signal in the selected communication channel, and hence will be free to issue an uplink frame of data on that selected communication channel. It should be further noted that this grant signal of preferred embodiments can be issued to the subscriber terminal on any selected communication channel in any frame, irrespective of whether that frame contains a data portion destined for that subscriber terminal.
In preferred embodiments the grant signal grants the subscriber terminal access to the selected communication channel to send one frame, the subscriber terminal being arranged to continue asserting the request signal until a grant signal has been received for the final frame that the subscriber terminal has to send.
In addition to the header portion having control fields to provide information about the data portion, and to provide grant signals to subscriber terminals for uplink communications, various other control fields can also be provided within the header portion. For example, in preferred embodiments, the header portion includes a power control field for identifying a power control signal to be used by the recipient of the frame to control the power of signals subsequently issued by that recipient. In preferred embodiments, the power control signal specifies incremental adjustments to be made to the power. Additionally, in preferred embodiments, the header portion includes a code synchronisation control field for identifying a code synchronisation signal to be used by the recipient of the frame to control the code synchronisation of signals subsequently issued by that recipient. Again, as with the power control signal, the code synchronisation signal preferably specifies incremental adjustments to be made to the code synchronisation.
It will be appreciated by those skilled in the art that to enable the field to be read accurately, the recipient of the frame needs to determine the phase of the carrier signal, which may vary over time. Accordingly, in preferred embodiments, the header portion includes a field containing a predetermined training sequence used by the recipient of the frame to determine the phase of a carrier signal.
It will be appreciated that the telecommunications system of the present invention may be either a wired or a wireless telecommunications system. However, in preferred embodiments, the telecommunications system is a wireless telecommunications system, wherein the transmission medium is a radio resource facilitating wireless communications between the central terminal and the subscriber terminal. Further, in preferred embodiments, the communication channels are orthogonal channels defined using CDMA.
Viewed from a second aspect, the present invention provides a method of operating a telecommunications system to route data of a plurality of different data types between a network and subscriber terminals of the telecommunications system, the subscriber terminals being connectable to a central terminal of the telecommunications system via a transmission medium, the telecommunications system providing a number of communication channels arranged to utilise the transmission medium for transmission of data between the central terminal and the subscriber terminals, the method comprising the steps of: providing first transmission processing logic for employing a first transport mechanism to transmit data and second transmission processing logic for employing a second transport mechanism to transmit data; routing data for transmission to either the first or second transmission processing logic dependent on first predetermined criteria, the first predetermined criteria comprising at least the data type of the data for transmission; and determining based on second predetermined criteria which of the communication channels to allocate for use by the first transmission processing logic and which of the communication channels to allocate for use by the second transmission processing logic.
Viewed from a third aspect, the present invention provides a computer program operable to configure a telecommunications system to perform a method in accordance with the second aspect of the present invention. The present invention also relates to a carrier medium comprising such a computer program.